1. Field of Invention
The present invention relates to a method of determining a color separation rule employable in an image processor such as a process scanner and an apparatus for converting image signals of a plurality of originals, and in particular, relates to determination of a signal-conversion characteristic defining a color separation rule employable in an image processor such as a process scanner.
2. Description of Prior Arts
A process color scanner which is disclosed in the U.S. Pat. No. 4,679,095, for example, is known in the field of image processors for obtaining halftone dot images (reproduced images) from original images. As shown in FIG. 14, the conventional process color scanner comprises an image scan reader 101, an image processing unit 102, an image scan recorder 103, a display device 104 and a data processor 105.
The image scan reader 101 reads the image of an original (not shown) mounted on an input drum (or an original table).
The image processing unit 102 comprises: a look-up table memory for storing a normalizing curve therein; a color computer for converting the Blue, Green and Red (R, G, B) components of image data into Yellow, Magenta, Cyan and Black (Y, M, C, K) components; and other circuits. The image processing unit 102 is operable to obtain input image data from the image scan reader 101 and to convert the input image data into output image data through the look-up table memory and the color computer. The normalizing curve defines a conversion characteristic for converting the range of the input image data or signals into a range appropriate to image recording by the image scan recorder 103.
The image scan recorder 103 comprises a dot generator for receiving the output image data from the image processing unit 102 and converting the same into halftone dot signals. In response to the halftone dot signals, a halftone dot image is recorded through photo-scan of a photosensitive material which is mounted on an output cylinder or is linearly moved.
The display device 104 has a CRT display, on which an image expressed by the output image data having been processed in the image processor 102 is displayed.
The data processor 105 comprises a console, a CPU, a memory and the like, and is operable to generate the normalizing curve to be set in the look-up table memory in the image processor 102 on the basis of the input image data obtained by the image scan reader 101. The data processor 105 can correct the normalizing curve and designate regions on the image displayed on the CRT of in the display device 104 in response to manual operation of the console.
According to a conventional technique for determining the normalizing curve in the image processor, an operator designates points on the original image to determine a highlight point and/or a shadow point, which will be hereinafter referred to as "reference density point(s)". The reference points are points through the normalizing curve passes, such the density levels of the pixels corresponding to the points are converted into predetermined levels.
FIG. 13 depicts an example of the normalizing curve. In the depicted example, the normalizing curve C is a straight line passing through two reference pints, i.e., a highlight point HL and a shadow point SD, which are provided on a coordinate plane defined by a horizontal axis representing input signals D.sub.X (X=B, G, R) and a vertical axis representing normalized density signals D.sub.NX (X=B, G, R).
The values D.sub.NX and D.sub.NH for the normalized density signals D.sub.NX are values common to all color components. The value D.sub.HX for the input signals is a highlight density value for each color component B, G, R, and the value D.sub.SX is a shadow density value for each color component B, G, R. The coordinates of the highlight point HL and a shadow point SD are (D.sub.HX, D.sub.NH) and (D.sub.SX, D.sub.NS), respectively. Accordingly, each input signal D.sub.X is converted by the normalizing curve C connecting the two points, whereby the normalized density signal D.sub.X which is in the range defined between the values D.sub.NH and D.sub.NS. The normalizing curve C is determined for each color component B, G, R to normalize each color component B, G, R, whereby an optimum color separation rule is obtained.
For the case where a plurality of original images are exposed on photo-films of a same type or under same conditions, which is the case where a plurality of original images are prepared at a same time for making a catalog of goods, for example, it is preferable that the plurality of original images are color-separated according to a same color separation rule to unify the respective tones of recorded images of respective original images. A conventional technique to determine a uniform or common color-separation condition such that a typical one of the plurality of original images is selected by an operator and color-separation condition is determined for the selected one original image in accordance with the process described above. The color separation rule thus determined is applied to color-separation of respective original images.
The conventional technique in which an operator selects one original image and the color-separation rule based only on the selected one original image is applied to respective original images has the following disadvantages:
(1) The selection of typical one image within a plurality of original images requires a skill fill operator and tends to reflect differences among individuals. Accordingly, it is difficult to always obtain desirable color separation rules.
(2) A plurality original images to be recorded sometimes include images in which color tones are deviated to highlight sides or shadow sides. Thus, it is unreasonable to apply a color-separation rule based only one original to all original images for all cases.